Many products which require sterilization, such as nutritionals and pharmaceuticals, have traditionally been packaged in glass containers. The technology associated with the sterilization of glass containers is very well developed. Glass bottles are most frequently sterilized under conditions in which there is a net vacuum inside the container so as not to subject the glass to tension during sterilization.
However, consumers have increasingly indicated a preference for plastic containers, due to factors such as lower cost, lower potential for container breakage with dangerous sharp debris, lower weight, and ecological concerns. In some instances a very hot liquid is placed into a plastic container during a "hot filling" operation and the plastic container is not subjected to retort conditions. However; for some products the plastic containers are filled, with a relatively cool liquid and then subjected to retort conditions to sterilize the contents. The sterilization of plastic containers has required careful control of sterilizer pressure in order to minimize excessive container deformation and the resulting catastrophic failure of such containers. In addition, the rate of change of sterilizer temperature has tended to be constrained by the need to minimize container-to-container temperature variations and thus the simultaneous need for different pressures for different containers within the sterilizer. Also, the maximum allowable container temperature has been limited due to a tendency of the plastic containers to become weaker at higher temperatures and a need for excessive pressures to prevent container deformation.
Typically, when containers are filled, steam is injected into the container just prior to the container being sealed. During sterilization, problems can arise with the deformation of a sealed container due in part to the inter-relatedness of product volume, headspace gas volume, and container volume. In a container packed without the use of a vacuum, the volume of product and the volume of the headspace gas equal the volume of the container. In a container packed under a vacuum, the volume of product plus the volume of the headspace gas is less than the volume of the sealed container, and the total fill equals the headspace volume plus the product volume.
The sterilization of plastic containers presents the possibility of encountering a problem herein referred to as catastrophic failure. Containers which experience catastrophic failure exhibit post-sterilization shapes which do not approximate the containers' pre-sterilization shape. If a failure occurs in the bottom of a container due to inadequate sterilizer pressure, the failure is called a buckled bottom or end. If a failure occurs in a sidewall of a container due to either inadequate or excessive sterilizer pressure, the failure is called a panel failure. Closure failure and failure of other container features are also common.
One proposed solution to the long felt need for a retortable plastic container is disclosed in U.S. Pat. No. 4,125,632. This patent proffers as the solution to the problem of catastrophic failure the presence of localized thin spots in the bottom wall of a container to facilitate expansion and contraction of the container's bottom during sterilization. This patent discloses that it is critical that the thickness of the sidewall must be thicker than the thickness of the base. Unfortunately, due to the criticality of the varying wall thickness the plastic container disclosed in U.S. Pat. No. 4,125,632, the can taught therein can only be made using certain manufacturing methods. For example, the container disclosed in the patent can not be made by extrusion blow molding.
Commonly owned U.S. patent application Ser. No. 817,001 filed on Jan. 3, 1992 discloses a retortable plastic container having a low panel strength and a bottom profile described by a particular equation. If a designer or engineer should choose to provide a container with features that result in a high panel strength such as using stronger plastics, using thick sidewalls or employing strengthening features such as ribs, catastrophic failures may still be frequently experienced. The teachings of this co-pending patent application still leave unsolved the problem of catastrophic failure during sterilization of a plastic container having a high panel strength.
As used herein and in the claims "panelling" is understood to mean a localized deformation in the sidewall of a container. As used herein and in the claims "panel strength" is understood to mean the net external pressure (difference between external and internal pressure) at which the sidewall of an empty sealed container buckles at a temperature of 21.3.degree. C. (70.degree. F.). As used herein and in the claims a "high panel strength" is understood to mean a panel strength of equal to or less than 17.5 kPa (2.54 p.s.i.). As used herein and in the claims a "low panel strength" is understood to mean a panel strength of equal to or less than 18.5 kPa (2.54 p.s.i.).
A critical performance requirement in retortable plastic containers with high panel strength is the capability of a container to deform in such a manner as to increase the volume of the container with increasing temperature and internal pressure, and decrease the volume of the container with decreasing temperature and internal pressure without experiencing a catastrophic failure. One benefit of a container possessing this capability is that with an increasing range of attainable container volumes during sterilization, the variation of the internal pressure in a container experienced during a given sterilization process is reduced. However, this capability also minimizes both the magnitude and range of internal pressures in containers during sterilization. The capabilities of a container to increase and decrease in volume reduces the possibility that either inadequate or excessive sterilizer pressure will cause a container to sustain a catastrophic container failure. Another benefit is that this capability also provides markedly larger allowable ranges of operating parameters which are ancillary to the sterilization process such as product fill, headspace gas volume, sterilizer pressure, product temperature, etc.
Containers which have the capability to expand a significant amount during sterilization and return substantially to their pre-sterilization shape without experiencing a catastrophic failure are easier to sterilize because such containers can survive diverse temperature-pressure conditions, thus allowing the use of rapid heating and cooling batch and continuous sterilizers, dependent on container fill conditions. Preferably a container must be able to deform to provide a container volume increase of at least about 6%, corresponding to the thermal expansion of the liquid packaged in the container, dependent on headspace gas volume, and preferably in excess of 10% without experiencing catastrophic failure of the container. This capability is especially advantageous when sterilizing heat sensitive nutritional and pharmaceutical products in which minimizing the thermal degradation of either product nutrition or medical potency is essential. Another coincident benefit is significantly reduced manufacturing costs due to higher sterilizer productivity. In a high panel strength container the majority of the expansion needs to occur in the bottom wall of the container, and a container in accordance with the invention disclosed herein has a recessed center portion which allows the required volume changes without panelling of the container.
A container structure which has utility for high panel strength retortable plastic containers is disclosed in co-pending U.S. patent application Ser. No. 702,558 filed May 20, 1991. It has been observed, however, that for both high and low panel strength retortable plastic containers the method of molding the container places limitations upon the dimensions and shape of the portion of the bottom of the container which is intended to compensate for changes in the volume of the container's contents during retort. The present invention provides high and low panel strength plastic containers which may be manufactured using a two piece mold which as used herein and in the claims is understood to be a mold which has two mating mold halves which contact one another along a plane which contains the longitudinal axis of the container. This plane along which the mold halves meet is referred to herein and in the claims as the "parting line" of the mold. The containers of the present invention are capable of the above described volume changes and may be manufactured using a two piece mold. The degree of volume change of which a plastic container of the present invention is capable cannot be attained with the structure taught in copending U.S. application 702,558 filed on May 20, 1991 without resorting to the use of at least a three piece mold in the manufacturing process. A three piece mold is one in which the bottom wall of the container is formed by one piece of the mold and the remainder of the container is formed by the other pieces of the mold. Therefore, the present invention facilitates a more economical method of manufacturing containers requiring fairly large volumetric changes during a retort procedure.